This invention relates to pacemaker apparatus for delivering stimulus signals to a patient's heart, and in particular pacemaker apparatus adapted to vary the rate of the delivered stimulus signals as a function of sensed physiological changes.
As is set forth in the referenced U.S. Pat. No. 4,228,803, for a patient that is being stimulated by a cardiac pacemaker there is a correlation between optimum heart rate and the stimulus-T wave (S-T) time interval. Briefly, this time interval carries information relating to the patient's physiological condition, which information indicates the rate at which the patient's heart would beat if it were beating naturally and without a pacemaker. The sensing of a shortened S-T interval corresponds to a desired higher rate, and the sensing of a longer S-T interval corresponds to a desired lower rate. By continuously detecting the S-T interval and translating it into a correlating control signal, the rate of the pacer can be varied to provide delivered stimulus pulses at a desired rate more closely corresponding to that called for physiologically.
It has been determined that other information is contained in the heartbeat signal, and particularly the parameters of the QRS wave and the T wave, which can be used as input information for deriving rate control signals. For example, our analysis indicates that there is a rate correlation with both the frequency spectrum and the amplitude of the T wave. Generally, we believe that the larger the frequency composition, indicating a sharper T wave, the higher the desired heart rate. Likewise, the larger the observed peak amplitude of the T wave, the larger the desired heart rate. Another T wave parameter which is related to frequency spectrum is that of time interval, the interval of the T wave between defined leading and trailing reference levels. The smaller this time interval, corresponding to a larger frequency spectrum, the higher the desired heart rate, and vice versa. Likewise, the characteristics of the QRS wave are believed to correlate in generally the same manner. Thus, there are a number of parameters of the T wave and QRS wave which can be sensed and examined in order to obtain information for generation of a control signal which can in turn be used to modify or vary the rate of the stimulus generator used in the pacemaker. This observation leads to the conclusion of monitoring evoked QRS and T waves to continuously, i.e., from heartbeat to heartbeat, generate control signals for continuously controlling the pacemaker output rate. Additionally, the parameters of natural heartbeats may likewise be monitored while the pacemaker is in the inhibited mode, in the event it is desirable to place the pacemaker into a fixed rate mode upon the occurrence of predetermined circumstances.
Another correlating parameter which may be monitored is that of the patient's threshold to delivered stimulus pulses. As is well known in the pacing literature, threshold is defined as the level of the stimulus pulse required to evoke a resulting heartbeat when the pulse is delivered to the patient's heart. It has been determined that there is a correlation between the patient's threshold and certain physiological conditions, such as exercise, eating and sleeping. See "Changes in Myocardial Threshold. Physiologic and Pharmacologic Factors In Patients With Implanted Pacemakers", Preston et al, American Heart Journal, Vol. 74, No. 2, pp. 235-242, August, 1967; "Threshold Studies in Transvenous Cardiac Pacemaker Treatment", Westerholm, Scandanavian Journal of Thoracic and Cardiovascular Surgery, Supplementum 8, 1971. It is noted that the desired cardiac output of normal human subjects likewise varies as a function of exercise, eating and sleeping, leading us to the conclusion that there is a correlation between the patient's threshold and desired heart rate. It is widely accepted that the increase in cardiac output of normal patients is mainly supported by an increment in heart rate. "Initial Adjustment of Cardiac Output in Response to Onset of Exercise in Patients With Chronic Pacemaking as Studied by the Measurement of Pulmonary Blood Flow", Koyama et al, American Heart Journal, April, 1976, Vol. 91, No. 4, pp. 457-459.
From the above, it is seen that the foundation of the invention is that of capturing information available in a patient's heart signals detected at the pacing situs, and utilizing that information in order to control the paced heart rate corresponding to the patient's physiological condition. A pacemaker arrangement which provides for analysis of the various parameters of the QRS and T wave portions of the heart rate, as well as means for continuously tracking pacing threshold, provides the desired information. Means is thus provided for an automatically rate adaptive pacing system which utilizes to a maximum extent the available pertinent information derived from the heartbeat signal which is sensed by the same electrode which is used to deliver the pacing signal to the patient's heart.